The field of the invention is industrial digital control systems, and particularly, programmable controllers such as that disclosed in U.S. Pat. No. 3,942,158 issued Mar. 2, 1976, and entitled "Programmable Logic Controller".
Programmable controllers are typically connected to industrial equipment such as assembly lines and machine tools to sequentially operate the equipment in accordance with a stored control program. In programmable controllers such as that disclosed in the above cited patent, for example, the control program is stored in a random access memory and includes instructions which are executed in rapid sequence to examine the condition of selected sensing devices on the controlled equipment and instructions which energize or deenergize selected operating devices on the controlled equipment contingent upon the status of one or more of the examined sensing devices.
Programmable controllers are distinguishable in both structure and application from process controllers which employ general purpose digital computers to control industrial equipment and processes. Whereas the primary function of a process controller may be to monitor a number of analog signals and control a number of analog operating devices, the primary function of programmable controllers is to monitor large numbers of single-bit digital devices, such as limit switches, and control a large number of single-bit operating devices such as solenoids and motor starters. As a result, the processor, or central logic unit of a programmable controller is typically less complex, less powerful and less costly than the processor of a general purpose computer, both because it processes single bits rather than words and because it is not called upon to perform substantial arithmetic functions. On the other hand, due to the large number of discrete sensing and operating devices connected to a programmable controller and the severe environment in which they are typically located, the I/O interface circuitry which couples industrial equipment to a programmable controller processor may be elaborate. In the above cited patent, for example, the I/O interface circuitry is coupled to the programmable controller memory through a unique input/output scanner circuit which operates asynchronously with respect to the controller processor to couple digital data between the electrically hostile industrial environment and the controller memory.
As programmable controllers grow in size and capability, they are finding increased application in industrial systems and processes which include analog devices and positioning devices which have heretofore been controlled by process controllers or special purpose hardwired control systems. Such devices are word oriented unlike the bit-oriented devices typically controlled by a programmable controller and special measures must, therefore, be taken to interface these word-oriented devices with a programmable controller.